This invention relates to a semiconductor memory, and more particularly to a technique which is effective when utilized for a memory having a built-in refresh circuit, such as a dynamic RAM (random access memory)
The memory cells of a dynamic memory are constructed of storage capacitors for storing data in the form of charges, and MOSFETs for selecting addresses. In the memory cells formed on a semiconductor substrate, the charges stored in the capacitors decrease with time on account of leakage currents, etc. In order to keep accurate data stored in the memory cells, therefore, the so-called refresh operation needs to be performed in which the data items stored in the memory cells are read out before vanishing, and the read data items are amplified and then written into the same memory cells again. By way of example, an automatic refresh circuit described in a magazine "Denshi Gijutsu (Electronics Technology)", Vol. 23, No. 3, pp. 30-33 has been known as an automatic refresh system for memory cells in a dynamic RAM of 64 kbits. The contents thereof are as follows. The dynamic RAM is provided with an external terminal for refresh control This dynamic RAM is furnished with an auto-refresh function according to which a refresh control signal REF of predetermined level is applied to the external terminal, whereby a plurality of memory cells in the dynamic RAM are automatically refreshed. In addition, it includes a self-refresh function according to which the refresh signal REF is held at the predetermined level, whereby the refresh operation is performed every fixed cycle in accordance with an increment pulse formed on the basis of pulses provided by a built-in timer circuit.
Since such an automatic refresh circuit subjects all the memory cells to the refresh operation in the same cycle, a very short refresh cycle of approximately 2 ms is selected in consideration of the worst case. The dynamic RAM continues to perform refresh operations at such very short time intervals, so that the greater part of the power consumption thereof is ascribable to the refresh operations.